1. Check ID of the menu URL ( e.g. Google Chrome - Inspect )



 


2. Check ID of the menu (here 240):



3. Add CSS code and modify it to your needs:


  • menu-item-ID
  • padding-left (space at the left of the link)
  • URL to your image
  • margin-top - controls vertical position


#menu-item-240 > a > .link_content > .link_text {
padding-left:30px!important;
position:relative!important;
}
 #menu-item-240 > a > .link_content > .link_text:before {
content: url(https://dummyimage.com/25x25/ff00ff/fff.jpg);
position: absolute;
left: 0;
margin-top: -5px;
}



4. In this article, you can find information how to add custom CSS to any WordPress Theme: How to add CSS code that is future update compatible


5. Final effect with sample purple 25x25 image